1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and devices used to control a hot restart of a centrifugal compressor driven by a low speed shaft of a gas turbine.
2. Description of the Prior Art
In natural gas processing systems, often a gas turbine 10 drives a centrifugal compressor 20. When a sudden shutdown occurs, a shaft (or rotor) of the centrifugal compressor 20 usually develops a thermal bow due to an asymmetric cooling (e.g., an upper side and a lower side of the centrifugal compressor 20 have different rates of cooling). If the cooling process is allowed to be completed, eventually, the thermal bow disappears. However, often it is necessary to restart the centrifugal compressor 20 sooner then the end of the cooling, while the thermal bow is still present. The restarting of the compressor 20 before the cooling is completed is named a hot restart.
When the centrifugal compressor 20 is restarted while the thermal bow is still present, vibrations may occur. The vibrations usually have a maximum amplitude at a speed lower than operational speeds. Strong vibrations may result in rubbing and permanent damage to the compressor 20.
A method of suppressing the thermal bow without waiting for complete cooling (which may last from several hours to several tens of hours depending on the compressor size and geometry) is to run the compressor 20 for a while at a low speed. This method is known as the slow roll method.
Conventionally, the slow roll method is implemented by inserting a turning gear 60 that may be attached to the compressor 20 (as shown in FIG. 1) or to a gearbox 30, if present. The turning gear 60 is used to rotate the shaft 50 of the compressor 20 at a low speed until the bow is suppressed.
However, adding a turning gear increases the cost of the system. Sometimes, a turning gear cannot be added to a system due to structural constraints. Additionally, the turning gear may fail, and, therefore, unintentionally, the compressor may be damaged by running at damaging speeds while still having a bow.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.